In recent years, the need for higher levels of cleanliness in clean rooms has increased in proportion to the degree in which semiconductors have been integrated and the capabilities of liquid crystals have improved, and thus air filter units having a higher particle collection efficiency are in demand.
Up until now, high performance air filters which are used in these types of air filter units, particularly HEPA (High Efficiency Particulate Air) filters, ULPA (Ultra Low Penetration Air) filters, and the like, are made by pleating a filter medium made of glass fibers in a wet paper forming process.
However, although the cost of the power needed to supply air from an air conditioner must be reduced, and it is desirable that the pressure loss of an air filter unit be reduced and the collection efficiency of the air filter unit be increased in order to achieve an even cleaner space, it is extremely difficult for an air filter unit formed from glass fibers to achieve higher levels of performance (higher collection efficiency if the pressure loss is the same, lower pressure loss if the collection efficiency is the same).
Accordingly, in order to produce an air filter unit having higher levels of performance, an air filter unit has been previously proposed which employs a porous film formed from polytetrafluoroethylene (hereinafter, PTFE) that has capabilities that are higher than a glass fiber medium. It has been reported that by employing a PTFE porous film, the pressure loss will be ⅔ that of an ULPA filter which employs a glass fiber medium having the same collection efficiency (Japanese unexamined patent application publication No. H05-202217, and PCT Publication Nos. WO94/16802 and WO98/26860).
In addition, although the capabilities of an air filter unit that employs a PTFE porous film can be further improved by the method used to produce or process the same, a PTFE porous film having improved capabilities has been previously proposed for use in this type of air filter unit. A simple high performance PTFE porous film (a filter medium that does not have an air permeable support member laminated thereto) is proposed in Japanese unexamined patent application publication H09-504737 (national phase of PCT application), Japanese unexamined patent application publication H10-30031, Japanese unexamined patent application publication H10-287759, and PCT Publication No. WO98/26860. These publications disclose a PTFE porous film having a high PF (Performance Factor) value (which indicates the capabilities of the filter medium). In addition, Japanese unexamined patent application publication H10-287759 discloses a PTFE porous film in which the highest PF value thereof was 32.
Up until now, it has been thought that the PF value of a PTFE porous film could be increased by reducing the diameter of the fibers in the PTFE porous film. However, according to the most recent research, even if the diameter of the fibers is reduced, there are limits to how much the PF value will be improved (O. Tanaka et al., Proceedings of the 15th ICCCS International Symposium, pp. 454-463). This is because when the diameter of the fibers of the PTFE porous film is reduced, the single fiber collection efficiency η of the film will indeed increase and exceed 1, but the actual single fiber collection efficiency η of the film will be lower than the calculated value of η due to interference from nearby fibers.
The cause of this will be explained with reference to this document (Proceedings of the 15th ICCCS International Symposium).
FIG. 15 shows the relationship between a fiber 61 in a conventional medium and particles 63 that are collected by the fiber 61, and FIG. 16 shows the relationship between fibers 71 that form a PTFE porous film and particles 73 that are collected by the fibers 71. In the figures, the diameter of the fibers 61, 71 that form the medium is df, and the width between particles positioned away from a fiber by a predetermined distance and collected thereby is de. In this situation, the single fiber collection efficiency η of the medium is expressed by:η=de/df  [Formula 1]
As shown in FIG. 15, in the conventional medium the fiber diameter df is comparatively large and the simple fiber collection efficiency η is less than 1, however the simple fiber collection efficiency η of the PTFE porous film will be greater than 1 because the fiber diameter df is much smaller.
Because of this, as shown in FIG. 16, the area S in which one fiber is capable of collecting particles partially overlaps with the area S′ in which another nearby fiber is capable of collecting particles (i.e., the area P shown in the figure), and thus the collection efficiency per fiber, i.e., the single fiber collection efficiency η will decline.
The result, as noted above, is that the actual single fiber collection efficiency of the PTFE porous film as a whole will have a value that is smaller than the theoretical single fiber collection efficiency (the single fiber collection efficiency that does not take the reduction of the collection efficiency produced by interference from other fibers into consideration).
Thus, it has been difficult to increase the PF value because the actual single fiber collection efficiency of a medium is not expected to increase even if the diameter of the fibers of the medium are reduced, and although a PTFE porous film having a PF value of 32 is known in the prior art, a PTFE porous film having a PF value that exceeds 32 has not been known in the prior art up until now.
In addition, from a practical point of view, an air permeable support member must be laminated to the PTFE porous film in order to use the film as a filter medium. This is necessary, from the point of view of handling, in order to increase the strength of the PTFE porous film and to protect the medium from damage when the medium is processed into desired shapes. However, when an air permeable support member is laminated to the PTFE porous film, the PTFE porous film will be compressed in the thickness direction thereof, the distance between the fibers of the PTFE porous film will be reduced thereby, and as a result the actual η value will be further reduced, and the PF value of the PTFE porous film to which an air permeable member is laminated will be lower than the PF value of a PTFE porous film alone.
In fact, Japanese unexamined patent application publication No. H10-30031 discloses that when an air permeable support member is laminated to a PTFE porous film having a PF value of 26.6, the PF value thereof will become 19.8. In the prior art, PCT Publication No. WO98/26860 discloses that a medium formed from a PTFE porous film laminated to an air permeable support member has a PF value of 21.8, and the Proceedings of the 15th ICCCS International Symposium discloses that this combination has a PF value of 28. However, a medium that exceeds these PF values is not yet known in the prior art.
A first object of the present invention is to provide a high performance filter medium formed by laminating an air permeable support member to at least one side of a PTFE porous film, and in which a decline in the single fiber collection efficiency η of the filter medium after heat lamination is controlled, the pressure loss of the filter medium is low, and the collection efficiency of the filter medium is high. More specifically, for example, it is an object to provide a high performance filter medium formed by laminating an air permeable support member to at least one side of a PTFE porous film, and in which a decline in the single fiber collection efficiency q of the filter medium after heat lamination is controlled, the pressure loss of the filter medium is low, and the collection efficiency of the filter medium is high, such that the actual single fiber collection efficiency η of the filter medium is 80% of the calculated single fiber collection efficiency η thereof.
A second object of the present invention is to provide a filter pack formed from a pleated filter medium formed by laminating an air permeable support member to a PTFE porous film, the filter pack having low pressure loss and high collection efficiency.
A third object of the present invention is to provide an air filter unit in which a filter pack formed from a pleated filter medium formed by laminating an air permeable support member to a PTFE porous film is accommodated inside a frame, the air filter unit having low pressure loss and high collection efficiency.
A fourth object of the present invention is to provide an efficient method of manufacturing the aforementioned filters.